Abstract: Iron deficiency in human infants and in young animal models produces changes in neural functioning that may be related to monoamine metabolism. This study employed both behavioral and biochemical approaches in a design using cross-fostering to examine alterations in dopamine (DA) function when iron deficiency occurs during the neonatal period. We measured brain Fe, dopamine transporters (DAT) and dopamine receptor density In rats made iron deficient, or not, from postnatal day (PND) 4 to PND 14 or 21. Some pups were then weaned to an iron-deficient diet and others to the control diet to examine the reversibility of these effects. Behaviors related to dopamine function were measured. Dopamine D2 receptor (D2R), D,R and iron concentrations were ~70, 80 and 30% of control values, respectively, in the nucleus accumbens and striatum in Iron-deficient rats at PND 14. The DAT density was also reduced to 50% of control density In the nucleus accumbens but was unchanged in the striatum. By PND 21, there was also a significant 50% lowering of DAT, D,R and D2R densities in the prefrontal cortex (PFC). Iron repletion at PND 21-49 normalized D1R, D2R, and DAT levels in the nucleus accumbens, PFC and ventral midbrain but not in the striatum. In summary, neonatal Iron deficiency is associated with changes in DA biology that vary with duration of iron deficiency, and are not completely normalized despite replenishment of iron status. Changes in DA-related behaviors that were persistent after postweaning iron repletion suggest the existence of a critical neonatal developmental period that is expressed by alterations in DA functioning. J. Nut, 133: 1174-1179, 2003.